Just a bunch of raw ideas. I really wish one day diaspora can replace facebook. Open and distributed social network means freedom. Cheers!
Usernames should be unique within the whole public diaspora network. This could be achieved by setting up directory servers handling the list of all users saying which pods are hosting them + one e-mail address for user verification purposes. Directory servers’ only purpose would be to store the users/pods/e-mails database (not passwords, since logon is pod distributed).
It could make the network more resilient in case of a pod failure, as the user would just have to choose another pod while keeping his/her username. It also makes it simpler to other users because reference to a user would now remain unchanged whatever the hosting pod (directory servers translating unique username to actual user@pod address).
It would also be easier for any user to choose a different pod spontaneously for whatever reason. Signing up to a different pod with existing username would have to go through e-mail verification and, if passed, would trigger deletion of user profile hosted by former pod. We can even imagine a direct pod-to-pod migration tool.
User should regularly backup his/her profile on a computer or cloud service. It should be strongly encouraged, using a backup scheduling tool perhaps. Backups would be restored by user in case of pod failure or for moving to another pod.